This is also the conclusion we had in this issue: FSI: Subcycling with implicit coupling · Issue #58 · precice/openfoam-adapter · GitHub
And here is the warning: https://github.com/precice/openfoam-adapter/blob/22568dc8c26cedf6f795bcf69927c63b307bd4c6/Adapter.C#L610
The adapter does not fully support subcycling for FSI and instabilities may occur.